Concept of Matter

Concept of Matter

Explain concept of matter

Matter is anything that has mass and occupies space. Therefore, anything around us provided it has mass and can occupy the space, is termed as matter. There are many kinds of matter. Can you mention some? The word matter is used to cover all the substances and materials from which the earth and universe is composed of. These include all materials around us such as water, soil, plants, animals, air, clothes, etc.

Any particular kind of matter is called a substance. Substances include elements and compounds. An element is a substance which is the limit of chemical analysis. When two or more elements are combined chemically, a compound is formed. Matter is made up of atoms, ions or molecules. You will learn more about this later.

States of matter

The Three States of Matter

Describe the three states of matter

Any chemical substance we study exists in any of the three forms (or physical states). The three different states of matter are

1. solid state
2. liquid state and
3. gaseous states

So, each of the many millions of substances around us can be classified as a solid, a liquid or gas. Look around you and name substances that are solids, liquids and gases. The state in which any matter exists depends on temperature and sometimes pressure conditions. One substance may exist as a solid in one condition and as a liquid or gas under a different condition. Water is an example of such substances. This change is called a change in the state of matter.

The three physical states of matter differ in the way they respond to temperature and pressure. All three states can increase in volume (expansion) when the temperature is increased. They decrease in volume (contraction) when the temperature is decreased. Gases are easily compressed. Liquids are only slightly compressible. Solids are incompressible. They are not affected by change in pressure.

Investigation of the compressibility of solids, liquids and gases

Procedure

1. Take three new syringes and fill them with sand, water and air respectively (figure 5.1).
2. Try to push in the end of each syringe.
3. Observe what happens.

Figure

Observation

Which of the substances under investigation can compress into a smaller volume?

Findings

You should have found that a solid (sand) and a liquid (water) cannot be compressed but a gas (air) is easily compressed.

The three states of matter differ in their physical properties. These differences in properties are summarized in table bellow

Differences in properties of the three states of matter

One State of Matter to Another

Change one state of matter to another

We have seen that matter exists in three different states - solids, liquids and gases. We can use the kinetic theory of matter to explain how a substance changes from one state to another. Basically, changes from one state to another are caused by alterations in temperature and pressure. Normally molecules, ions or atoms of a substance move faster when the temperature is increased.

Melting and freezing

Melting is a change from solid to liquid state. When solids are heated, their constituent particles (atoms, molecules or ions) get energy and vibrate more violently. Vibrations of these particles overcome (exceed) their binding forces. The particles become mobile. The crystalline structure of solid is destroyed. A liquid state is reached and the particles are free to move. The temperature at which this happens is called melting point of the solid.

The melting point of a solid tells us something about the strength of forces holding its constituent particles together. Substances with high melting points have strong forces between their particles. Those with low melting points have weak forces between their particles.

Freezing is a change from liquid to solid state. Freezing is the opposite of melting. The process is reversed at the same temperature if a liquid is cooled. The temperature at which a substance turns to a solid is called freezing point. The melting point and freezing point of any given substance are both the same. For example, the melting and freezing of pure water takes place at 0°C. Melting is not affected by any changes in atmospheric pressure.

Evaporation and boiling

Boiling is a change from liquid to vapour state at a particular temperature. Evaporation is the change from liquid to vapour state at any given temperature. If a liquid is exposed to open air, it evaporates. Splashes of water evaporate at room temperature. After rain, small pools of water dry up. When a liquid changes into a gas at any temperature, the process is called evaporation. Evaporation takes places from the surface of the liquid. The larger the surface area, the faster the liquid evaporates. The warmer the liquid is, the faster it evaporates. Thus, surface area and temperature affects the rate of evaporation of a liquid.

When a liquid is heated, its molecules get more energy and move faster. They knock into each other violently and bounce further apart. As the heating goes on, its molecules vibrate even faster. Bubbles of gas (due to air dissolved in water) appear inside the liquid. The whole process is called boiling. The temperature at which a liquid boils is called boiling point.

The molecules at the surface of the liquid gain enough energy to overcome the forces holding them together. They break away from the liquid and from a gas (vapour). As more of the liquid molecules escape to form a gas, a liquid is said to evaporate. This occurs at the boiling point of a liquid.

The temperature at which a liquid boils explains how strong the forces holding its particles (molecules) together are. Liquids with high boiling points have strong forces of attraction between their molecules than those liquids with low boiling points.

The boiling point of a liquid can change if the surrounding pressure changes. If the surrounding pressure falls, the boiling point also falls. The boiling point of water at standard pressure (760 mmHg) is 100°C. On a high mountain, where pressure is low, it is lower than 100°C. If the surrounding pressure is increased, the boiling point rises. The same behaviour is experienced by a gas when the pressure is either increased or decreased.

The melting and boiling points of some common chemical substances at standard temperature and pressure (s.t.p)

From the above explanation, obvious differences between evaporation and boiling can be detected. See table bellow

Therefore, the two terms can be defined as follows: Evaporation is a change in state of a substance from liquid to gas (vapour) state at any temperature.

Boiling is a change in state of a substance from liquid to gas at a particular temperature and pressure.

Condensation and solidification

The reverse of evaporation is condensation. This is brought about by cooling. When a gas is cooled down, its particles lose energy. They move more and more slowly. When they knock into each other, they do not have enough energy to bounce away again. They stay close together and a liquid forms. This process is called condensation. When the liquid is cooled further, the movement of the particles slows down even more. Eventually, they stop moving and a solid forms. This is called solidification.

Condensation can be defined as a change in state of a substance from gas (vapour) to liquid. Solidification is a change from liquid to solid state of a substance. Solidification is the same as freezing.

Sublimation

A few solids do not melt when they are heated. Instead, they change directly from the solid to gaseous state without passing through the liquid state. This change in state is called sublimation. When a solid changes directly into gas, it is said to sublime. Iodine, solid carbon dioxide ("dry ice") and ammonium chloride are examples of solids that sublime. Like melting, sublimation also occurs at one particular temperature for each pure solid.

The Importance of Changing One State of Matter to Another

Explain the importance of changing one state of matter to another

The following points summarize the importance of change in state:

1. Separation of mixtures

Different mixtures can be separated through such processes as distillation, sublimation, evaporation and condensation. Let us have a look at an example of distillation. This process involves boiling, evaporation and condensation. Distillation as a process can be applied in separation of a mixture (solution) of two or more substances. A mixture of two or more substances with different boiling points e.g. water and alcohol can be separated by this means. In such a case, a container with the mixed-up liquids is heated. The liquid with a low boiling point evaporates and condenses first, leaving the one with a high boiling point in the container. The distillate (liquid with low boiling point) is collected, cooled down and transferred into another container.

2. Industrial manufacture of products

Industrially, the process of distillation is applied in the production of pure substances such as beer and other alcoholic drinks such as wine, vodka, konyagi, etc. The manufacturing process involves boiling, evaporating and condensation.

3. Refining of petroleum (crude oil)

Crude oil contains organic liquid components, each with a different boiling point. In the refinery, the components with lower boiling points evaporate first and get separated out, leaving those with higher boiling points behind. In this way, we get various types of oil components (fractions) such as petrol, diesel, kerosene, lubricating oil, etc.

4. Drying of crops and clothes

When you suspend your clothing on a cloth line to dry, the moisture in it is lost through evaporation. Likewise, farmers in the village often spread crops on the ground to dry. They do this in order to reduce moisture content and hence prevent decaying. The moisture contained in crops leave by evaporation. Therefore, you can notice how evaporation, as a change in state, is important in everyday lives.

5. Cooling of our bodies in hot weather

You all like to drink cold water or beverages especially during hot weather. You can use a refrigerator to cool down drinking water or beverages directly. Alternatively, you can freeze water into ice and then use the resulting ice for cooling the beverage. Ice blocks are also saleable. Moreover, one can earn some money if she freezes water into ice blocks and then sells them to beverage vendors. Perishable products such as fish, meat, milk, etc are often packed in ice blocks to prevent them from going bad. Ice, as we studied early, is formed when water freezes (a change in state from liquid to solid).

6. Ice formation in refrigerators

You all like to drink cold water or beverages especially during hot weather. You can use a refrigerator to cool down drinking water or beverages directly. Alternatively, you can freeze water into ice and then use the resulting ice for cooling the beverage. Ice blocks are also saleable. Moreover, one can earn some money if she freezes water into ice blocks and then sells them to beverage vendors. Perishable products such as fish, meat, milk, etc are often packed in ice blocks to prevent them from going bad. Ice, as we studied early, is formed when water freezes (a change in state from liquid to solid).

7. Melting metals to make alloys

In metallurgical industries, need may arise to mix two or more metals (alloys) together. This is only possible, where two or more metals are first melted at high temperatures into liquids. Then the resulting liquid metals are mixed in appropriate proportions. This is followed by cooling down the mixture to a solid alloy. Normally alloys have better qualities than individual metals.

8. Testing the purity of substances

The presence of impurity may lower or raise the boiling point of the substance. A pure substance melts and boils at definite temperatures (see table 5.4). The values for the melting point and boiling point are precise and predictable. This means that we can use them to test the purity of a sample. They can also be used to check the identity of unknown substance.

A typical example

Sea water is impure. It freezes at a temperature well below the freezing point of pure water (0°C) and boils at a temperature above the boiling point of pure water (100°C). Other substances behave in a similar manner. So, boiling as a change in state can be used to test for the purity of a substance.

In addition, the impurity also reduces the exactness of the melting or boiling point. An impure substance melts or boils over a range of temperature, not at a particular point.

Substance	Melting point (°C)	Boiling point (°C)
Water	0	100
Ethanol	-117	78
Oxygen	-219	-183
Sodium	98	890
Sulphur	119	445
Iron	1540	2900
Diamond	3550	4832
Cobalt	1492	2900
Nitrogen	-210	-196
Propane	-188	- 42
Ethanoic acid	16	118

9. Formation of rain

Perhaps the most important of all, as far as change in state is concerned, is the formation of rain. Rained is mainly formed through the process of evaporation and condensation. Water vapour, evaporating mostly from water bodies (oceans, seas, lakes, rivers, ponds, etc), land and plants rises up to the sky. As it rises, it cools down and condenses into tiny droplets

On further cooling as they rise up, these droplets form bigger water drops. Owing to gravitational force, these drops fall down as rainfall. Every one of you knows how important rain is to our life. Therefore, you have noticed how evaporation and condensation, as changes in state, contribute to rain formation.

Kinetic nature of matter

We already know that matter is composed of atoms, ions or molecules. We have not yet considered the reason why the same substance, say water, can exist in more than one form, for example as solid ice, liquid water, and gaseous steam. But does matter behave like that?

The kinetic theory of matter has been used to explain the way in which the arrangement of the particles of a substance can determine the properties of that substance, and particularly the state in which it is likely to be found under a given set of conditions. The idea is that all matter is made up of tiny moving particles. The main points of the theory are as follows:

1. All matter is made up of tiny particles (atoms and molecules) that are invisible to the naked eye and to most microscopes.
2. The particles are moving all the time. The higher the temperature is, the higher the average energy of the particles.
3. Heavier particles move more slowly than lighter particles at the same temperature.
4. Each substance has unique particles that are different from the particles of other substances.
5. The particles of matter are held together by strong electrostatic forces.
6. There are empty spaces between the particles of matter that are very large compared to the particles themselves.

The solid state

In the solid state, the particles are so closely packed (see figure bellow. The particles are held together by strong forces of attraction that act like a chemical glue. Free movement of particles cannot take place. They cannot move around freely in this arrangement. Instead, they vibrate about a fixed position. They are arranged in a fixed pattern which form a cluster of vibrating masses. This makes a solid to have a fixed shape, which cannot be changed except by applying strong external forces.

The liquid state

The particles of a liquid are also closely packed but the forces of attraction between them are weaker than of a solid. These forces of attraction tend to bind them together. The particles have more kinetic energy and they can move around each other. The binding forces are strong when particles come close to one another. It is thought that the particles of a liquid are fairly randomly arranged but consist of "clusters" closely packed together. This property makes a liquid to have a definite volume. However, since the particles are fairly free to move a liquid does not have any characteristic shape (see figure 5.5(b). Thus, a liquid will always take the shape of its container.

The gaseous state

The gaseous state is one in which the particles are moving independently of each other in all directions and at great speeds. The particles of a gas are relatively far apart. They exert no force of attraction on each other. They have more energy than the particles of solids and liquids. They move rapidly and randomly, colliding with each other and with the walls of the container. A typical speed for a molecule of hydrogen in air at ordinary temperature and pressure has been found to be approximately 500 ms-1

It has been estimated that a nitrogen molecule makes collisions each second. Thus, a gas will rapidly spread out to fill any container in which it is placed. A gas cannot have any shape of its own.

Physical and chemical changes

Depending on the nature of change, all changes that matter undergoes can be classified as either physical or chemical.

The Characteristics of a Physical Change

Describe the characteristics of a physical change

Physical change

Substances may undergo changes in their physical properties e.g. changes in colour, shape (or form), state, density, structure and texture, etc. If you take a stone and break it down into small particles, you will have only changed its form, but it will remain as a stone. Likewise, melting ice to water or freezing water to ice does not change it, but it is still water. The same case happens when you dissolve salt in water to get a solution of salt in water. You can still get back the original salt by evaporation, except that the crystals of the salt obtained will not look exactly the same as those of the original salt.

These changes of state are examples of physical changes. Physical changes such as melting and boiling do not result in new substances being formed

For example, ice and water still contain the same particles whether in solid (ice) liquid (water) or gaseous (vapour) state.

Characteristics

In the explanation above, we find that in a physical change it is only the physical form, and not the actual nature, of a substance that changes. The changes are brought about by a mere addition or removal of heat, as in the case with water or ice. Such a change is called a physical change. It can be distinguished by the following characteristics:

1. There is no formation of a new substance. Consider an example given above. The ice, liquid water and steam are the solid, liquid and gaseous forms of the same substance (water).
2. There is no change in weight of the substance undergoing the change. If you start with 50g of ice, you will still get the same mass of water and steam (vapour) upon melting and boiling respectively.
3. The changes are readily reversible. You can easily change water back to ice and vapour to water by a mere subtraction of heat (cooling).
4. It is not accompanied by a great heat change. Just a little heat is required to change ice to water, and water to steam.

Physical Changes of Matter Experimentally

Demonstrate physical changes of matter experimentally

Experiment

1. Add some common salt (sodium chloride) to distilled water in a beaker. Stir the mixture until the salt disappears and forms a solution with water. Transfer the water into a porcelain dish. Heat the content until all the water has evaporated off. The salt reappears in its original white solid form.
2. Grind some roll sulphur in a mortar to powder. Put the resultant powder in a test-tube and heat gently, shaking all the time. The sulphur melts to an amber-coloured liquid. On cooling, this liquid returns to its original condition as a yellow solid.
3. Put a block of ice in a beaker. Heat gently until the whole block melts to form water. Pour the water formed in a cup and place it in a deep freezer overnight. The water will freeze back to ice.

You will have seen that all the above changes involve only changes in physical forms of the substances. The chemical nature of substances remained unchanged. Therefore, we can define a physical change as a change that does not involve formation of a new substance but involves a change in state or physical form of the substance and that such a form can be reversed.

The Characteristics of a Chemical Change

Describe the characteristics of a chemical change

Chemical change

Some changes that materials undergo are permanent. Such changes usually involve changes in chemical properties of a substance. For example, when you burn a piece of wood in fire, you get ash. The properties of wood and ash are very different. There is no way you can change ash back to wood. It is practically impossible. A permanent change in chemical properties of a substance is called a chemical change. In a chemical change, a substance losses all its physical and chemical properties.

Characteristics

Includes

1. A chemical change results in the formation of a new substance. The new substance has different chemical and physical properties as compared to the original substance.
2. It is generally not reversible. For example, you cannot turn the ash back to wood.
3. There is a change in weight or mass of the substance undergoing the change. When you burn wood weighing 5 kg, you cannot expect to get the same weight of ash.
4. The change is accompanied by a considerable heat change. For wood to burn to ash a lot of heat must be supplied.

Chemical Changes of Matter Experimentally

Demonstrate chemical changes of matter experimentally

Experiment

1. Strongly heat some roll sulphur on a deflagrating spoon until it melts and begins to burn with a blue flame. If you continue heating, it gradually decreases in amount and finally the spoon will be left empty. The disappearance of sulphur is due to the formation of a new gaseous substance that is invisible. The presence and existence of a gas in air can be defected by its irritating smell. The gas can also be detected by burning the sulphur in a gas jar to which some blue litmus solution has been added. The gas formed, sulphur dioxide, will turn the blue litmus paper into a red one.
2. With the aid of tongs, subject a piece of magnesium ribbon to a Bunsen burner flame. The ribbon burns to produce a new substance, white ash of magnesium oxide.
3. Wrap a wet cotton wool around an iron nail. Keep it in a test tube for 3 days. By the 3rd day, some brown marks of rust will appear on the surface of the nail. Rust is hydrated iron (III) oxide. This is quite a new substance compared to iron nails.

Table

Physical change	Chemical change
1. Produces no new kind of matter	Always produces a new kind of matter
2. There is no change is mass or weight of the substance	2. There is a substantial change in the weight of the substance
3. The change can be reversed	3. The change cannot be reversed
4. Little heat is absorbed or evolved	4. Heat changes may be large
5. The change involves only a change in physical properties of a substance	5. Both physical and chemical properties are changed.

Elements and symbols

The Concept of an Element

Explain the concept of an element

Our world and universe are made up of millions of different substances. We have already seen how these substances can be classified into solids, liquids and gases. However, on close examination, we find that these substances are made up of a number of small elements.

An element can be defined as a substance that cannot, by any known chemical process, be split into two or more simpler substances. This means that elements cannot, by any chemical process, be made to yield substances simpler than themselves. An element is a substance because it has the same composition throughout.

In 1803, a scientist called John Dalton suggested that each element was made up of its own kind of particles. He called these particles the ‘atoms’. Therefore, an element is a substance that is made up of only one kind of atoms.

Names and Symbols of Elements

A chemical symbol is the way of representing an element using initial letter(s). There are many different elements as you have seen above. Every element has a name and a symbol to represent it. Some symbols are just a single capital letter, such as H. Others have two letters, the first of which is always a capital, such as Mg.

Rules for assigning chemical symbols to elements

Includes

1. Each element is given a different symbol to represent it.
2. Some elements are represented by two letters e.g. Ca (for calcium), Cl (for chlorine), etc.
3. If two letters represent the element, the first letter is always a capital and the second letter is always a small letter e.g. argon (Ar) and helium (He).
4. In order to avoid confusion, some elements have their chemical symbols derived from Latin names

All symbols are recognized and are used by all scientists all over the world. Some examples of elements and their symbols are given in the table below

Element	Symbol	Element	Symbol
Aluminium	Al	Bromine	Br
Copper	Cu	Carbon	C
Iron	Fe	Chlorine	Cl
Lead	Pb	Hydrogen	H
Magnesium	Mg	Nitrogen	N
Mercury	Hg	Oxygen	O
Potassium	K	Phosphorus	P
Silver	Ag	Sulphur	S
Sodium	Na	Silicon	Si
Calcium	Ca	Iodine	I
Manganese	Mn	Fluorine	F
Tin	Sn	Gold	Au
Chromium	Cr
Zinc	Zn
Nickel	Ni

t is easy to remember that the symbol for aluminium is Al, and for carbon is C. But some symbols are harder to remember because they are taken from Latin names. For example, potassium has the symbol, K from its Latin name Kalium. Sodium has the symbol, Na from its Latin name Natrium. See the complete list in the following table.

English name	Latin name	Chemical symbol
Sodium	Natrium	Na
Gold	Aurum	Au
Potassium	Kalium	K
Mercury	Hydragyrum	Hg
Silver	Argentum	Ag
Antimony	Stibium	Sb
Lead	Plumbum	Pb
Tin	Stannum	Sn
Iron	Ferrum	Fe
Copper	Cuprum	Cu
Tungsten	Wolfram	W

The reason for assigning some elements with Latin names was to avoid confusion among scientists when representing different elements. For example, the symbol for silicon is Si. It could be impossible to represent silicon by the symbol S and at the same time represent the element sulphur by the very symbol, S. Similarly, potassium could not be represented by the symbol P that was assigned to phosphorus. So in order to avoid such confusion, scientists decided to use Latin names to represent some elements. In so doing, the anticipated and unnecessary contradiction among scientists from different parts of the world was avoided.

Symbols are particularly useful when more than one atom is present in a substance. For example, hydrogen gas consists of pairs of hydrogen atoms joined together. So hydrogen gas is shown as H2. When more than one atom is joined together like this, we call the substance formed a molecule. Atoms making up gases such as hydrogen, oxygen, nitrogen, etc, always exist as molecules. Sulphur exists as a hexagonal ring of eight atoms. Phosphorus exists as a tetrahedron of four atoms. Table bellow shows some elements that exist as molecules.

Element	Atomic symbol	Molecular symbol
Oxygen	O
Nitrogen	N
Hydrogen	H
Sulphur	S
Phosphorus	P
Chlorine	Cl
Fluorine	F
Bromine	Br
Iodine	I

Classification of Elements

Elements can be classified as either metals or non-metals. Metals and non-metals have different physical and chemical properties. This is the criterion used for classification of these elements into metals on one hand and non-metals on the other hand. Table bellow summarizes the physical and chemical properties of some common elements.

Element	Date of discovery	Metal or non-metal?	Solid,liquid or gas?	Melting point (°C)	Boiling point (°C)	Density ( g cm-3)
Oxygen	1774	Non-metal	Gas	-219	-183	0.00132
Nitrogen	1772	Non-metal	Gas	-210	-196	0.00117
Carbon	Ancient	Non-metal	Solid	3500	4827	22
Iron	1735	Metal	Solid	1540	3000	7.9
Copper	Ancient	Metal	Solid	1080	2500	9.0
Lead	Ancient	Metal	Solid	327	1744	113
Gold	Ancient	Metal	Solid	1060	2700	193
Silver	Ancient	Metal	Solid	961	2200	10.5
Hydrogen	1766	Non-metal	Gas	-259	-253	0.00008
Aluminium	1825	Metal	Solid	660	2450	27
Zinc	1746	Metal	Solid	419	910	7.1
Mercury	Ancient	Metal	Liquid	-39	357	13.6
Iodine	1811	Non-metal	Gas	114	183	4.9
Chlorine	1774	Non-metal	Gas	-101	-35	0.003
Sodium	1807	Metal	Solid	98	890	0.97
Potassium	1807	Metal	Solid	64	760	0.86
Sulphur	Ancient	Non-metal	Solid	119	444	2.1
Phosphorus	1669	Non-metal	Solid	44	280	1.8

Metals and non-metals

There are 94 naturally occurring elements. Some of them are very rare. Francium, for instance, has never been seen. The radioactive metals neptunium and plutonium, which we make artificially in quite large amounts, only occur in very small (trace) quantities naturally. Most of the elements can be classified as metals. The rest are non-metals. To understand these elements better, refer to the Periodic Table of Elements at the back of this book. Classification of elements into metals and non-metals is based on differences between their physical and chemical properties. Differences between metals and non-metals are shown in table bellow.

The differences between Metals and Non-metals

A. Physical properties

Mixtures	Compounds
1. The components of a mixture can be separated by physical means, e.g. filtering, magnetic separation, decantation, etc	The components of a compound can be separated by chemical means only
2. The composition of a mixture can vary widely, e.g. a mixture of 20g of sand with 1g of salt or vice versa.	Compounds are fixed in their compositions by mass of elements present, e.g. there are always 2 atoms of hydrogen to 1 atom of oxygen in a molecule of water
3. Mixing is not usually accompanied by external effects such as explosion, evolution of heat, or volume change (for gases)	Chemical combination is usually accompanied by one or more of these effects
4. Properties of a mixture are the sum of the properties of the individual constituents of the mixture.	The properties of a compound are quite different from those of its constituent elements. For example, water is a liquid whereas its constituent elements, hydrogen and oxygen, are both gases.
5. No new substance is produced as the mixture forms	A new substance is always produced when a compound forms.

B. Chemical Properties

1. Give basic oxides, that is, oxides which react with acids	Give acidic oxides, that is, oxides which react with bases
2. Replace hydrogen in acids to form salts	Do not react with acids in this manner
3. Form positive (+) ions	Form negative (-) ions
4. Form electrovalent chlorides which are stable in water	Form covalent chlorides which react with water.
5. Do not react with hydrogen	Form stable compound with hydrogen

Elements from Other Substances

Differentiate elements from other substances

There are over 105 different elements known. Of these, 90 have been obtained from the Earth's crust and the atmosphere, and 15 have been artificially made by scientists. From this small band of elements, all other substances on earth are made. Table 5.6 shows the approximate percentage composition by mass of the elements in the earth's crust, the oceans, and the atmosphere. Can you notice the abundance of oxygen? Analysis of the earth's crust, the oceans, and the atmosphere, reveals that oxygen is the most abundant element on earth, accounting for half the total mass.

Percentage by mass of elements in the earth’s crust	Percentage by mass of elements in the oceans	Percentage by mass of elements in the atmosphere
Oxygen	47	Oxygen	86	Nitrogen	75.5
Silicon	28	Nitrogen	10.9	Oxygen	23
Aluminium	7.8	Chlorine	1.8	Argon	1.4
Iron	4.5	Sodium	1.0	Hydrogen	0.02
Calcium	3.5	Magnesium	0.1	Carbon	0.01
Sodium	2.5	Calcium	0.05	Others( total)	0.07
Potassium	2.5	Sulphur	0.05
Magnesium	2.0	Potassium	0.04
Titanium	0.5	Nitrogen	0.02
Hydrogen	0.2	Bromine	0.01
Carbon	0.2	Carbon	0.01
Others( total)	1.3	Others (total)	0.02

Compounds and mixtures

Compounds and Mixtures

Concept of compounds and mixtures

A compound is a substance that contains two or more elements chemically combined together. A mixture is something that contains two or more elements not combined chemically. It is always difficult to identify a mixture from a compound. Before going any further into this topic, let us start by looking at the differences between compounds and mixtures. These differences are summarized in the table below.

Differences between mixtures and compounds

Mixtures	Compounds
1. The components of a mixture can be separated by physical means, e.g. filtering, magnetic separation, decantation, etc	The components of a compound can be separated by chemical means only
2. The composition of a mixture can vary widely, e.g. a mixture of 20g of sand with 1g of salt or vice versa.	Compounds are fixed in their compositions by mass of elements present, e.g. there are always 2 atoms of hydrogen to 1 atom of oxygen in a molecule of water
3. Mixing is not usually accompanied by external effects such as explosion, evolution of heat, or volume change (for gases)	Chemical combination is usually accompanied by one or more of these effects
4. Properties of a mixture are the sum of the properties of the individual constituents of the mixture.	The properties of a compound are quite different from those of its constituent elements. For example, water is a liquid whereas its constituent elements, hydrogen and oxygen, are both gases.
5. No new substance is produced as the mixture forms	A new substance is always produced when a compound forms.

A Binary Compound

Prepare a binary compound

A compound is a substance that contains two or more elements chemically combined together. This is a very important difference from mixtures. Mixtures can contain more than one element but the elements are not chemically combined. The number of chemical substances known is approximately four millions. All compounds on earth are made from about one hundred simple materials. Such compounds range from simplest substances, like water, which contains only two elements, to those complex materials of which our own bodily tissues are composed. The following is a short list of common compounds and the elements they are made of.

The Properties of a Compound with those of its Constituent Elements

Compare the properties of a compound with those of its constituent elements

Elemental composition of some compounds

Compound	Constituent elements
Water	hydrogen and oxygen
Carbon dioxide	carbon and oxygen
Ethanol	carbon, hydrogen and oxygen
Sugar (sucrose)	oxygen, hydrogen and carbon
Sodium chloride(common salt)	sodium and chlorine
Marble (calcium carbonate)	calcium, carbon and oxygen
Sulphuric acid	hydrogen, sulphur and oxygen
Sand

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